In recent years, solvents used for printing inks have been changing because of reinforcement of legislation, and an increasing interest in environmental protection and safety. In Japan, an outbreak of cholangiocarcinoma in 2012 has made people interested in safety of solvents used in printing work.
As solvents of printing inks for plastic films, aromatic solvents such as toluene and xylene have been conventionally used. However, from the viewpoint of safety, uses of toluene are encouraged to be avoided, and non-aromatic solvents such as ketone solvents and ester solvents started to be used instead. In recent years, from the viewpoint of toxic environments of printing work, uses of ketone solvents have been encouraged to be avoided also, and thus uses of ester solvents and alcohol solvents have been the current mainstreams. In particular, from the viewpoint of environmental protection and safety of work environments, an ink, in which an alcohol solvent such as ethanol, isopropanol, n-propanol, and methanol is used as a main component, has been desired.
Polyolefin base materials such as polypropylene have superior properties and are cheap on prices, and thus they have been widely used as a variety of films such as food wrapping materials; auto parts; and molded products. However, since the polyolefin base material is a nonpolar base material having the low surface free energy, it is one of low-adherent base materials. Thus when printing or coating is performed to a polyolefin base material, it has been generally known that adherence can be improved by adding chlorinated polyolefin into an ink and paint.
However, chlorinated polyolefin is hard to be dissolved in an alcohol solvent that has been increasingly used in the field of inks as mentioned above. In Patent Literature 1, it is described that a graft copolymer that is obtained by grafting an acryl monomer having an ethylenic unsaturated bond to chlorinated polyolefin having a given weight-average molecular weight and a given percentage of a chlorine content is used as a component of a resin composition for a printing ink not using an aromatic solvent such as toluene.
A urethane resin (polyurethane resin) has the following characteristics: tough, and a wide range of physical properties from soft to hard can be obtained; easy to control molecular weights and structures; easy to modify the molecules; excellent adherence to a variety of base materials; excellent chemical resistance and wear resistance; and excellent flexibility under low temperatures. Thus, the urethane resin is widely used as a binder for inks.
Although the urethane resin is somewhat adherent to both of polar base materials such as metals and nonpolar base materials such as polyolefin resins, stronger adhesiveness has been demanded. Meanwhile, a chlorinated polyolefin resin has excellent adhesiveness to polar base materials and nonpolar base materials, and is highly compatible with a variety of solvents and components. Accordingly, a chlorinated polyolefin resin is generally contained as an auxiliary binder in a urethane resin-based ink. However, chlorine contained in the chlorinated polyolefin is eliminated as hydrochloric acid by external factors such as heat and light, and this is the problem of the chlorinated polyolefin resin.
In Patent Literature 2, it is disclosed that a block copolymer including a polymer block (A) mainly comprising an olefin monomer unit; and a polymer block (B) including 2 to 100 mol % of vinyl monomer units having carboxy groups or anhydrous carboxylic acid groups and 98 to 0 mol % of other vinyl monomer units copolymerizable with the vinyl monomers may prevent the above-mentioned problem.